In relation to machine tools for performing a cutting process, there has been proposed various machining apparatuses and machining methods to perform a process that could not be done conventionally. One of the examples is a vibration cutting process in which a tool is reciprocated for a rotation-cutting process at a high speed to form a non-complete round shape.
According to a conventional rotation-cutting process, there is used a machine that includes a main shaft for rotating a workpiece and a rectilinear shaft for moving a tool bit, which is an example of a tool, in the radial direction of the workpiece. The tool is moved to a commanded position on the rectilinear shaft and is brought into contact with the workpiece being rotated, and thereby the sectional shape of the working object is machined into a complete round shape. At this time, if, while the workpiece is rotated, the tool is moved in the radial direction by reciprocating the tool on the rectilinear shaft, the workpiece can be machined into a non-complete round shape. For example, if the tool is reciprocated twice on the rectilinear shaft toward the center of the workpiece while the workpiece is rotated once, the sectional shape of the working object can be made into an elliptical shape. Specifically, the elliptical shape is formed such that its minor axis is defined by a distance between the tip of the tool and the center of the rotary shaft obtained at the time when the tool is positioned closest to the center of the workpiece, and such that its major axis is defined by a distance between the tip of the tool and the center of the rotary shaft obtained at the time when the tool is positioned farthest from the center of the workpiece. Further, the workpiece can be machined into a more complicated shape by deliberately adjusting the reciprocating motion pattern.
Further, Patent Literature 1 discloses a vibration cutting apparatus and a vibration cutting method that can machine a working object into a non-complete round shape. According to a technique disclosed in Patent Literature 1, a machine having rectilinear shafts of two axes or more is used to machine a working object into a non-complete round shape by giving periodic motion commands with different phase and amplitude to two shafts of the rectilinear shafts.
As described above, when a working object is machined into a non-complete round shape, a rectilinear shaft makes a sine wave-like motion which may contain a harmonic component depending on the curvature of the commanded machining shape. On the other hand, a motor control unit for a machine tool requires position control with higher speed and higher accuracy. However, if the feeding speed is set higher to increase the throughput rate or if the control gain is set higher to improve the accuracy, machine natural vibration is excited and machine resonance is thereby caused. Consequently the behavior becomes vibratory and the machining accuracy is lowered.
Conventionally, in consideration of the above, a filter for treating a position command signal is used to remove the machine natural vibration. The filter used is generally called “band elimination filter” or “low-pass filter” and is set to remove a specific frequency band component with respect to the position command signal or another state quantity inside the control unit, thereby preventing excitation of the machine natural vibration.
If the machine resonance is caused by a specific frequency, a band elimination filter for attenuating a frequency band component including this frequency is operated onto the position command signal. Here, if the frequency band is narrow, a notch filter can be used in place of the band elimination filter. Further, if the machine resonance is caused by a frequency of not less than a specific value and the frequency varies depending on the mass or another condition of a workpiece, a low-pass filter for attenuating the frequency component higher than the frequency is operated onto the position command signal. When a filter for treating the position command signal in this way is used, it becomes possible to realize a process with higher speed and higher accuracy without excitations of the machine resonances. In general, a frequency component contained in the position command signal is far lower than the cutoff frequency of a band elimination filter, notch filter, or low-pass filter that are operated thereon, so that the process for a commanded shape can be performed even if the filter is operated thereon.